Although located directly on the Loire, Nantes and its chateau is generally considered outside the region that encompasses the "chateaux of the Loire". Still, it is a wide walled complex of several buildings overlooking the river. Pierre Commarmond's poster shows the largest of the residences as seen from within the castle's courtyard.
Commarmond painted for several French railway companies, including some good ones for PLM (Paris-Lyon-Meditarrenee), the company with the best posters -- best because a) they had some of the best locations to work with (the Alps, the Riviera, Italy, North Africa, and the Holy Land), and b) because they had perhaps the best and most prolific railway poster artist of that era or any other, Roger Broders.
The Chateau of Nantes was originally home to the Duke of Brittany, and was therefore home to Anne of Brittany, who brought Brittany under the French crown when she married Charles VIII in 1491 and then married his heir Louis XII when Charles died young in 1498. It was also the site of the signing of the Edict of Nantes, issued by Henri IV in 1598 to bring the Huguenot wars to an end (repealed less than a century later by Louis XIV).